1. Field of the Invention
The present invention relates generally to a dynamic pressure type fluid bearing device suited for use in a laser printer and, particularly, in a motor of a scanner having a polygon mirror which requires highly accurate and high speed rotation.
2. Description of Related Art
FIG. 3 depicts one of the dynamic pressure type fluid bearing devices suited for use in high speed rotation, as disclosed in Japanese Laid-open Patent Publication (unexamined) No. 3-163212. The dynamic pressure type fluid bearing device as shown therein comprises a shaft 1, a hollow sleeve 2 in which the shaft 1 is rotatably inserted, and two sets of dynamic pressure generating grooves 8 and 9 formed in either the outer surface of the shaft 1 or the inner surface of the sleeve 2. This dynamic pressure type fluid bearing device has a space 7 defined between the two sets of dynamic pressure generating grooves 8 and 9. Air trapped in such a space 7 expands under the influence of heat generated during high speed rotation and then escapes outside. In order to easily and smoothly discharge the air, the dynamic pressure generating grooves 8 and 9 have hitherto been formed so as to have an acute angle of less than 20 degrees relative to the direction of rotation of the shaft 1 and a depth of more than 4 .mu.m so that the resistance to flow may be reduced. This construction prevents the air from remaining inside the bearing device during rotation and attains a stable rotation of the shaft 1.
However, all the dynamic pressure type fluid bearing devices do not have a completely well-balanced shaft. High speed rotation of an unbalanced shaft tends to cause whirling of the shaft which in turn brings about the so-called squeezing action in which the pressure of a lubricating oil is increased and causes the lubricating oil to escape from the bearing device. Because the acute angle of the dynamic pressure generating grooves 8 and 9 reduces the dynamic pressure generated within the bearing device and, hence, is disadvantageous to the squeezing action, the conventional dynamic pressure type fluid bearing devices are not satisfactory in terms of durability.
Furthermore, it has become known that when the circumferential speed of the shaft exceeds 1 m/s, air discharge is hindered under the influence of the centrifugal force of the lubricating oil as the dynamic pressure generating grooves 8 and 9 become deep.